linux-kernel - Re: [PATCH] [RFC] arm64: enable HAVE_LD_DEAD_CODE_DATA

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Message-ID: <2npq25ps-q960-795-96s5-rq6736pq693q@syhkavp.arg>
Date:   Wed, 10 Mar 2021 17:08:36 -0500 (EST)
From:   Nicolas Pitre <nico@...xnic.net>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
cc:     kernel-toolchains@...r.kernel.org,
        Masahiro Yamada <masahiroy@...nel.org>,
        Nicholas Piggin <npiggin@...il.com>,
        Fangrui Song <maskray@...gle.com>,
        Ard Biesheuvel <ardb@...nel.org>,
        Andrew Scull <ascull@...gle.com>,
        Mark Brown <broonie@...nel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        clang-built-linux <clang-built-linux@...glegroups.com>,
        David Brazdil <dbrazdil@...gle.com>,
        Geert Uytterhoeven <geert+renesas@...der.be>,
        Ionela Voinescu <ionela.voinescu@....com>,
        Kees Cook <keescook@...omium.org>,
        Kristina Martsenko <kristina.martsenko@....com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Mark Rutland <mark.rutland@....com>,
        Marc Zyngier <maz@...nel.org>,
        Nathan Chancellor <nathan@...nel.org>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        Will Deacon <will@...nel.org>, Arnd Bergmann <arnd@...nel.org>
Subject: Re: [PATCH] [RFC] arm64: enable HAVE_LD_DEAD_CODE_DATA_ELIMINATION

On Wed, 10 Mar 2021, Nick Desaulniers wrote:

> On Wed, Mar 10, 2021 at 1:08 PM Arnd Bergmann <arnd@...nel.org> wrote:
> >
> > On Wed, Mar 10, 2021 at 9:50 PM Masahiro Yamada <masahiroy@...nel.org> wrote:
> > > On Mon, Mar 1, 2021 at 10:11 AM Nicholas Piggin <npiggin@...il.com> wrote:
> > > > Excerpts from Arnd Bergmann's message of February 27, 2021 7:49 pm:
> >
> > >
> > > masahiro@...ar:~/ref/linux$ echo  'void this_func_is_unused(void) {}'
> > > >>  kernel/cpu.c
> > > masahiro@...ar:~/ref/linux$ export
> > > CROSS_COMPILE=/home/masahiro/tools/powerpc-10.1.0/bin/powerpc-linux-
> > > masahiro@...ar:~/ref/linux$ make ARCH=powerpc  defconfig
> > > masahiro@...ar:~/ref/linux$ ./scripts/config  -e EXPERT
> > > masahiro@...ar:~/ref/linux$ ./scripts/config  -e LD_DEAD_CODE_DATA_ELIMINATION
> > > masahiro@...ar:~/ref/linux$
> > > ~/tools/powerpc-10.1.0/bin/powerpc-linux-nm -n  vmlinux | grep
> > > this_func
> > > c000000000170560 T .this_func_is_unused
> > > c000000001d8d560 D this_func_is_unused
> > > masahiro@...ar:~/ref/linux$ grep DEAD_CODE_ .config
> > > CONFIG_HAVE_LD_DEAD_CODE_DATA_ELIMINATION=y
> > > CONFIG_LD_DEAD_CODE_DATA_ELIMINATION=y
> > >
> > >
> > > If I remember correctly,
> > > LD_DEAD_CODE_DATA_ELIMINATION dropped unused functions
> > > when I tried it last time.
> > >
> > >
> > > I also tried arm64 with a HAVE_LD_DEAD_CODE_DATA_ELIMINATION hack.
> > > The result was the same.
> > >
> > >
> > >
> > > Am I missing something?
> >
> > It's possible that it only works in combination with CLANG_LTO now
> > because something broke. I definitely saw a reduction in kernel
> > size when both options are enabled, but did not try a simple test
> > case like you did.
> >
> > Maybe some other reference gets created that prevents the function
> > from being garbage-collected unless that other option is removed
> > as well?
> 
> I wish the linker had a debug flag that could let developers discover
> the decisions it made during --gc-sections as to why certain symbols
> were retained/kept or not.

The GNU LD has --print-gc-sections to list those sections that were 
dropped. And normally you should be able to find why a section wasn't 
dropped by looking for dependencies in the linker map file.


Nicolas